Process for the chemical oxidation of aluminum and alloys thereof



Patented Aug. 29, 1950 PROCESS FOR THE CHEDIICAL OXIDATION OF ALUMINUIVIAND ALLOYS THEREOF Carlo Sonnino, Milan, Italy No Drawing. ApplicationMay 29,1947, Serial No. 751,455. In Switzerland October 6, 1944 Section1, Public m 690, August a. 1946 Patent expires October 6, 1964 Thisinvention relates to a process for the chemical oxidation of aluminumand its alloys whereby a colorless coating may be produced.

As already known, the most widely spread process for the chemicaloxidation of aluminum and alloys thereof, called MBV process, produces alayer consisting of approximately 73% aluminum hydroxide and 25%chromium hydroxide, the latter adding a dark coloring to the layer. Thiscoloring is a serious drawback for certain applications.

Various attempts have been made in order to eliminate this drawback ofthe MBV process and. to obtain colorless and transparent layers onaluminum and alloys thereof.

It was noticed that an addition of small quantities of sodium silicateto the solutions of sodium carbonate and sodium chromate used in thechemical oxidation speeded up the hydrolysis of the sodium aluminateformed during the oxidation and reduced considerably the quantity ofchromium hydroxide present in the layer, making it of a much lightercolor. This method is however very diificult, since even a slight excessof sodium silicate is sufficient to stop the reaction almost completely,giving a very poor and hardly protective oxide coating, and in siliconcontaining alloys producing no light colored layers.

It was therefore suggested to eliminate most of the chromium hydroxidefrom the layer by adding to the sodium carbonate and sodium chromatesolutions some sodium fluoride, which reacts with chromium hydroxideaccording to the following reaction:

and therefore eliminates the chromium hydroxide and makes the oxidelayer much more transparent.

It was however noticed that, although on aluminum and certain alloysthereof a very light colored layer is indeed obtained, the layer isnevertheless still rather grey on other alloys.

It was noticed that said coloring was due to the presence of graphiticsilicon and hydroxides of metals present in the alloy. In particular,the silicon is only eliminated through the action of fluorine or of itsacids, which are very active in the presence of oxidants, as forinstance sodium chromate.

The inventor of the present process had already noticed the interestingproperties of hydrofluosilicic acid (HzsiFs) and of its alkaline salts,which have a more regular and efficient 5 Claims. (01. 148-6.2)

2, action than sodium silicate upon the attack of aluminum and alloysthereof in alkaline solutions, while they reduce the violence of suchattack and speed up the hydrolysis of sodium aluminate.

These properties are utilized in accordance with the present inventionto make more regular the action of sodium carbonate and sodium chromatesolutions for the chemical oxidation of aluminum and its alloys, and. atthe same time to utilize the action of fluorine on the chromiumhydroxide of the layer, so as to transform it into soluble chromiumfluoride, and on the graphitic silicon so as to make it soluble as wellin the form of fluoride compounds.

In practice, the addition of 0.1% HzSiFc to the salts for the chemicaloxidation has proved sufficient, the sodium carbonate and sodiumchromate mixture being 5:15, in order to obtain with commercial aluminuma perfectly colorless and transparent layer, which has however aremarkable thickness.

For silicon alloys, the hydrofluosilicic acid addition should beconsiderably increased, depending on the silicon percentage, and afurther addition of an alkaline hydrofiuosilicate is also useful. Thesame is true for copper, manganese and magnesium alloys, where the bestresults are obtained both in color and thickness of the layer.

In practice, quantities varying from 0.1 to 10% by weight ofhydrofluosilicic acid HzSiFs are added to a solution of sodium carbonateand sodium chromate in the ratio 5: 1.5, either alone or in addition to0.1 to 10% of alkaline hydrofluosilicates. In certain cases an additionOf alkaline hydrofiuosilicates in percentages from 0.1 to 15% issuflicient. The solution thus prepared is heated to 95-100 C. and thearticles of aluminum or alloys thereof that are to be oxidized areimmersed in the solution for a period varying from 10 to minutes,depending on the desired thickness and on the nature of the alloy.

The articles are then washed and treated for 20 minutes with a boilingsolution of sodium silicate in order to fill the pores of the layer.

The layers thus obtained are perfectly colorless and transparent, have aremarkably higher thickness and resistance against corrosion than thoseobtained through other processes.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is:

11 A process for the chemical oxidation of aluminum and its alloys whichcomprises treating the metal to be oxidized in a hot bath consisting ofan aqueous solution consisting of a major proportion of sodium chromateand sodium carbonate and a minor proportion comprising at least 0.1% ofa compound selected from the class consistinggof liydifofiuosilicicacid; alkaline hydro: fiuosilicatesand mixtures of hydrofluosilicic'aeidand alkali fiuosilicates.

2. Process according to claim 1 wherein the solution comprises from0.15'130.10%*h'ydl.0fll'10.-

silicic acid.

3. Process according to claim 1 wherein the solution comprises from 0.1to 15% alkaline-hydrofluosilicates.

4. Process according to claim;.1; whereinlthe solution comprises from0.1 to 10% hydrofluosilicic acid and from 0.1 to 10 alkaline hydrofluosilicates.

5. Process according to claim 1, whereinthe metal is treated for aperiodof from 5 to 60 min- 20 utes in a bath maintained at a temperatureof from 95 to 100 C.

CARLO SONNINO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number: Name Date 1,723,067" Pacz Aug. 6, 1929,205,708., Zurbrugg June 25, 1940 2,247,580 Jauch July 1, 1941 2276353Thompson Mar. 17, 1942 2,296,884 Thompson Sept. 29, 1942 2,312,855.Thompson Mar. 2, 1943 FOREIGN PATENTS Number: Country Date 415,636 ItalyOct. 28, 1946

1. A PROCESS FOR THE CHEMICAL OXIDATION OF ALUMINUM AND ITS ALLOYS WHICHCOMPRISES TREATING THE METAL TO BE OXIDIZED IN A HOT BATH CONSISTING OFAN AQUEOUS SOLUTION CONSISTING OF A MAJOR PROPORTION OF SODIUM CHROMATEAND SODIUM CARBONATE AND A MINOR PROPORTION COMPRISING AT LEAST 0.1% OFA COMPOUND SELECTED FROM THE CLASS CONSISTING OF HYDROFLUOSILICIC ACID,ALKALINE HYDROFLUOSILICATES AND MIXTURES OF HYDROFLUOSILICIC ACID ANDALKALI FLUOSILICATES.